Since the first industrial revolution, fasteners have been developed rapidly, and played a very important role in the boom of industry. However, all the existing fasteners have a fatal weakness, i.e., it would automatically loosen in severe vibration. Threads are indispensable in the modern society. Loosening of a fastener may cause damage and disintegration of a part of a device or a complete device, and more significantly, lead to a major safety accident. For example, US Boeing Company, by investigation, found that one of causes of frequent accidents of 737 passenger planes is fall-off of a bolt of an aircraft wing. During US Sports Aviation Expo in Sebring, Fla., USA, a fatal crash happened to a Remos GX airplane, and the crash was caused by unfastening of a fastener of an aileron, resulting in one photographer dead and a pilot seriously wounded. It was reported from JR West Japan Railway Company that because an aluminum casing fell off and flew away due to its screw being loose, the Train “Sakura No. 561”, from Shin-Osaka to Kagoshima-chuo, hit the train, and went straight to power lines, causing short circuit and power failure.
Since fasteners appeared, scientists and engineers of many countries in the world have made numerous experiments and researches to delay the time of auto-loosening of the fasteners to a certain extent with a variety of methods, but the problem is not solved fundamentally. In the upcoming fourth industrial revolution, there will be higher demands for fasteners undoubtedly.
The loosening of threads is mainly due to a lateral clearance presented in engagement of the threads. When there is vibration, a basic element, “lead angle of thread”, of the thread becomes the “loose angle of thread”. In US, it has once proposed that a wedge-shaped thread, having a 30° angle with an axis, is made at a sharp corner of an inner tooth. Such a thread indeed improved the self-locking ability of the thread. But it can be seen from stress analysis that the thread has a great side force when stressed, especially at the initial stage of the stress. Local stress of a single tooth pattern is quite large, causing un-uniform stresses of teeth of the thread, therefore, the recycling of the fasteners has short life.